Abstract
Acanthomorph teleosts (spiny-rayed fishes) account for approximately a third of extant vertebrate species. They appeared during the Late Cretaceous and have been a major component of aquatic biodiversity since the early Cenozoic. They occupy today most trophic levels and ecological niches in aquatic environments, however very little is known about those that were adopted by the earliest representatives of the group. Here, we report on an exceptional glimpse into the ecological diversity of early spiny-rayed fishes provided by the unusual preservation of a newly discovered specimen of the freshwater acanthomorph Spinocaudichthys from the Upper Cretaceous of Morocco. A combination of major-to-trace elemental mapping methods reveals that the gross morphology of the specimen’s intestine has been remarkably preserved owing to the rapid mineralization of iron hydroxides around it. Differing with the typically short and straight intestinal tract of carnivorous teleosts, the intestine in Spinocaudichthys is long and highly convoluted, indicating a probable herbivorous diet. Acanthomorphs would therefore have conquered various ecological niches in their early evolutionary history, prior to their subsequent phylogenetic diversification in both marine and freshwater environments that followed the K-Pg extinction event.
Highlights
Spiny-rayed fishes of the clade Acanthomorpha are a major component of modern aquatic ecosystems
Elongated acanthomorph fish which differs from other acanthomorphs by the following combination of primitive and derived character states: 32 to 46 vertebrae; epineurals and epipleurals; no ossified supraneural; dorsal fin inserting posterior to neural spine 10 to 12; four spines and eight soft rays in dorsal fin; three spines and five soft rays in anal fin; ten dorsal and eight ventral procurrent spines in caudal fin; 20 principal rays in caudal fin; long neural spine of the second preural centrum; two ural centra unfused to the hypurals; two epurals; six unfused hypurals; two unfused uroneurals; free pelvic girdle inserted posterior to the postcleithra; pectoral fins low on the flanks; no spine in the pelvic and pectoral fins
In order to understand the different mode of exceptional preservation of the intestinal tract in MHNM-KK-OT 09, and to uncover any potential biases likely to affect the robustness of further palaeobiological and palaeoecological reconstructions inferred from this fossil, we explored its chemical composition with a combination of major-to-trace elemental mapping methods: synchrotron-based micro X-ray fluorescence and energy-dispersive X-ray spectroscopy (SEM-EDX)
Summary
Spiny-rayed fishes of the clade Acanthomorpha are a major component of modern aquatic ecosystems They account for approximately a third of extant vertebrate species, existing in every aquatic environments and at every trophic level[1,2]. They are predominantly found in oceans today and their early evolutionary history is known from many well-preserved fossil taxa found in marine outcrops of the Cenomanian (early Late Cretaceous) that document the appearance in the fossil record of most major modern acanthomorph groups[3,4]. This work offers an unexpected window into the palaeoecology of the earliest known freshwater acanthomorph representative, providing valuable insight on the ecological diversity of the group in the earliest steps of their successful evolutionary history
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